1. Technical Field
The present invention relates to radiometric imaging receivers and, more particularly, to implementations of microwave/millimeter-wave radiometers on silicon integrated circuits.
2. Description of the Related Art
Microwave and millimeter-wave radiometers, or radiometric imaging receivers, are used for a wide variety of purposes. Microwave radiometers were originally developed for radio astronomy, and have also been used for industrial and medical temperature measurement. Recently, there has been increasing interest in such radiometers for security and medical imaging. Traditionally, microwave radiometers have been implemented using relatively expensive microwave and millimeter-wave receiving techniques.
One of the most popular architectures for imaging receivers is the direct-detection receiver architecture, as shown in FIG. 1. The imaging receiver provides at its output an estimation of the antenna temperature by averaging the statistical noise fluctuations at the antenna. Unfortunately, fluctuations in receiver gain can cause relatively large variations at the receiver output, masking the desired output produced by noise fluctuations alone.
For the architecture shown in FIG. 1, the minimum detectable temperature difference (ΔT) between an antenna temperature TA and a known reference temperature TC is given by
                                                        Δ              ⁢                                                          ⁢              T                                      T              sys                                =                      2            ⁢                                                            1                                      B                    ⁢                                                                                  ⁢                    τ                                                  +                                                      (                                                                  Δ                        ⁢                                                                                                  ⁢                        G                                            G                                        )                                    2                                                                    ,                            (        1        )            where Tsys is the overall system temperature at the receiver input, B is the receiver bandwidth, G is receiver gain, and τ is the integration time. ΔT can be made largely independent of receiver gain variations by switching the receiver input between the antenna and a known reference load at some frequency fM using a Single Pole Double Throw (SPDT) switch, known as a Dicke switch or Dicke modulator.
In FIG. 1, an SPDT Dicke switch 14 is placed in front of the amplifier 15. The switch 14 takes as inputs first a signal from antenna 11, which comes mixed with some amount of noise 12, and second a resistor load 13. A reference generator 16 feeds into a switch driver 17 to rapidly oscillate the switch 14. Finally, a phase detector 18 takes the output of the amplifier 15 and the reference generator 16 to produce an output.
Unfortunately, the placement of the switch 14 in front of the amplifier 15 results in an increased receiver noise figure, or equivalently an increase in Tsys, deteriorating the imaging resolution (corresponding to a higher ΔT in equation (1)). Several solutions to this problem have been presented.
One such attempted solution utilized a balanced topology. However, such prior art architectures used components which are difficult to implement on silicon. The prior art architectures required at least four couplers, components which are very lossy when implemented on a silicon substrate. Furthermore, the couplers required an accurate 50Ω or open-circuit termination to function as intended. It is difficult to provide such terminations at millimeter-wave frequencies using silicon metal-oxide-semiconductor field-effect transistors (MOSFETs) or heterojunction bipolar transistor (HBTs), and each additional coupler effectively increases the noise. Further designs have used hybrid ring couplers, but such designs have lacked a balanced topology.